Types of Engines
There are a number of descriptions used for different types of steam engines Single vs. Double Acting In a single acting cylinder steam pressure is only applied to one side of the piston. An advantage of this arrangement is that an internal combustion engine type trunk piston can be used. The trunk piston functions as a crosshead, and the distance from the crankshaft to the cylinder head is considerably smaller allowing the engine to fit better in some spaces, for instance, under the hood of a car. Double acting engines have steam pressure applied alternately to both sides of the piston. This allows the piston to have two power strokes per revolution, More parts are necessary, namely, a crosshead, piston rod and rod packing gland (seal). Counterflow vs. Uniflow Engines can also be differentiated by the flow of the exhaust steam. In counterflow engines the exhaust port is either shared with the admission port or is at the same end of the cylinder. Hot steam is admitted to that end of the cylinder, it cools as it expands, then the cool exhaust steam is pushed out the same end of the cylinder. The steam alternately heats and cools the metal of the cylinder. This heat going into and out of the metal does no work and evenutally leaves with the exhaust. To make matters even worse, some of the admitted steam condenses on the cool metal, causing a great loss of heat energy. This condensed steam is known as the "Missing Quantity". Uniflow (the older name is Unaflow) engines have exhaust ports as holes in the cylinder wall near the bottom of the cylinder. The piston exposes these holes near the bottom of the stroke, allowing cylinder steam to flow to the exhaust. Exhaust steam does not travel back to the admission end of the cylinder, so it does not cool the cylinder surfaces. The admission end stays hotter, exhaust end stays cooler and less heat energy is lost heating and cooling the metal. Uniflow engines are typically more efficient than counterflow engines. They also do not need any exhaust valve mechanism. Their exhaust ports, however, typically close sooner than an exhaust valve would close, trapping more steam and causing more work to be done to compress it. This reduces the amount of work availble from the engine. Non-Condensing vs. Condensing If the exhaust steam is released to the atmosphere the engine is known as non-condensing (even though the steam condenses in the atmosphere). A condensing engine, not surprisingly, uses a condenser to cool the exhaust steam and condense it back to a liquid. One benefit is that water need not be replaced, and the condensate water will be quite pure. Often a greater benefit is that by condensing the steam below 212F (100C) the pressure will drop to below atmospheric. This increases the amount that steam can be expanded in the engine and increases its efficiency. Simple vs. Compound Steam engines can be made smaller and more efficient by employing higher pressure steam. Efficiency is lost, however, if all of the work potential is not captured by fully expanding the steam. Expanding steam once in a single cylinder before exhausting it is known as simple expansion. Practical limits exist for how much steam can be expanded in a cylinder. Ideally the high pressure steam will be expanded down to the exhaust pressure, extracting the maximum amount of work along the way. The volume of the steam can change dramatically while this happens. For instance, steam at 1000 psi and 800F (metric equiv.) exhausted at atmospheric pressure (14.7 psia) will have an expansion ratio of 39.6 to 1. Only a tiny amount of steam (about 2.5%, assuming no clearance) can be admitted. The cylinder will be quite cool overall, so condensation loses will be large. A large cylinder will be necessary to produce a modest amount of power. If a condenser was used to condense the steam to 126F and 2 psia, the necessary expansion ratio would be 257:1. In a compound engine the total expansion is divided among two or more cylinders. For instance, a two stage expansion from 1000 psi and 800F to 90 psi and 331F then expanded to atmospheric and 212F would have a much more practical expansion ratio of 6.3 in each stage. Engines on ships and stationary plants with cooling water that allowed steam to be condensed to quite high vacuums were built with three, four and even five expansion stages. Such engines were only justified in large engines and when condenser capacity allowed high vacuum operation. Self Starting and Reversing An engine will be self starting if it can be relied on to start rotating in the proper direction once the throttle valve is opened. This requires multiple cylinders placed at different angular positions around the crankshaft. The shorter the maximum cutoff allowed by the valve gear, the more cylinders are required. The valve gear may also allow enough adjustment to reverse the direction of rotation of the engine. If the engine is not self starting, the engine may be manually positioned with a piston just past TDC before the throttle valve is opened, or a source of initial rotation, such as an electric starter motor, is necessary.